This invention relates to the field of ozone synthesis. It presents a new operating process and apparatus for producing ozone from an oxygen-containing gas on a continuous basis, as required for many industrial oxidation demands, such as manufacture of peroxides and organic acids, treatment of municipal sewage and manufacturing wastes, large-scale disinfection and odor control applications, bleaching and treatment of potable water.
While various techniques are known for producing ozone on a laboratory scale, it is generally recognized that electrical corona is most suitable for industrial production of ozone, usually by the "silent electrical discharge" method. Ozone generators operating on this principle generally consist of two electrodes (or a plurality of such electrode sets), at least one of which is covered with a dielectric plate to define a discharge space. The electrodes are connected to a potential which produces an electric current accompanied by a pale bluish-violet light in the discharge space. Oxygen or a gas mixed with oxygen flows through the discharge space, wherein collision with impinging electrons provides the energy necessary for ozone formation. The dielectric acts to distribute and stabilize the discharge, thereby preventing sparking. Conventional ozone production is a function of several variables, including the flow rates of the throughput gases, the magnitude and frequency of the voltage present between plates, and the pressure temperature and composition of the gas.
Electrical efficiency of prior art ozone generators has been low. Since power cost is one of the major expenses in ozone production, any increase in efficiency is significant. Typically, the utilization efficiency of the power input is only about 10 percent; the other 90 percent of the energy is converted to heat, which decomposes ozone and lowers the efficiency. Avoiding ozone decomposition is a major object of this invention.
In ozone generators the heating problem may be overcome partially by water cooling. However, such cooling has only limited effect, because it only provides cooling at a surface, and not within the middle of the discharge space. Also, water cooling is impractical for an ungrounded electrode.
Recovery of freshly-generated ozone from a corona generator can be achieved in several ways. One of the more successful methods employs a selective adsorbent which removes ozone from the exit gas.
An adsorption unit for recovering ozone and recycling oxygen has been described by Kiffer in U.S. Pat. No. 2,872,397. According to this prior art system oxygen is converted into ozone in an electric discharge reactor and the resulting gas mixture containing a major fraction of oxygen and a minor fraction of ozone is passed through a bed of adsorbent particles, such as silica gel. The ozone is adsorbed by silica gel and the remaining oxygen is passed through the bed for recovery and recycle. The recycle stream is replenished continuously or periodically with oxygen in an amount to compensate for that which is converted to ozone product in the generator. Typically, a sufficient number of adsorption units are employed so that when the adsorbent silica gel in one vessel reaches the saturation point for ozone capacity, the ozone containing gas may be switched to other adsorption units. While the ozone-containing oxygen-rich stream from the ozone generator is being treated in the second adsorption unit, the ozone in the first unit is being removed from the adsorbent using a stripping gas such as air, nitrogen or other diluent gas which acts as a carrier for the ozone. This results in a safe ozone-carrier gas mixture suitable for subsequent chemical reaction without explosion hazards inherent in oxygen-ozone mixtures. The adsorption temperatures according to prior art methods may be about -80.degree.C to +20.degree.C. Since the adsorption capacity of silica gel decreases markedly as temperature approaches ambient, refrigerant cooling has been employed to increase the bed capacity. It is also known that the equilibrium of ozone between the adsorbent and gas phase is a function of ozone partial pressure. Accordingly, prior art attempts to employ silica gel adsorbent in ozone synthesis processes has favored high O.sub.3 concentration, high pressure and low temperatures.